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Water, Limnology and Flood Risk Assessments

Water and Flood Risk Assessments

The water assessment component of the EIS includes assessments of the project’s impacts on groundwater, surface water, reservoirs, and flood risk assessments by evaluating water quality and flow following guidelines set by the Water Quality Objectives (WQO) provided by NSW Water Quality and River Flow Objectives, which are consistent with ANZECC/ARMCANZ (2000). Additionally, the project is following regulations set by the Water Management Act 2000 and the NSW Aquifer Interference Policy (AIP). 

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Baseline assessments of water quality and flow for the project were taken prior to construction by groundwater and surface water monitoring networks. These networks were developed in consultation with the NSW Department of Planning, Industry and Environment (DPIE) hydrogeologists, hydrologists, and assessment officers.

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Impacts were assessed via simulation by creating models where there was no management or mitigation and assuming significant permeability for Gooandra Volcanics and Kellys Plain Volcanics underground, thus resulting in models that ‘overestimated’ the potential impacts of the project. However, the models did not simulate unprecedented geologic formations (such as fractures, which allow groundwater inflow), as per the Australian Groundwater Modelling Guidelines. 

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Significant predicted impacts of the project are expected to remain localized, and regional impacts are considered to be insignificant. No high priority groundwater-dependent ecosystems (GDEs) are expected to be impacted. Predicted impacts of the project include:

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  • groundwater drawdown and inflow into the tunnel, mostly during construction

  • decline in baseflow and streamflow to Gooandra Creek and Eucumbene River during construction and operation, particularly during dry climatic conditions, due to groundwater drawdown

  • the decline in watercourse water quality due to stormwater discharge in the first 1.5 years of construction

  • the decline in reservoir water quality due to wastewater discharge, stormwater discharge and rock plug removal during construction

  • the decline in reservoir water quality due to excavated rock placement (via erosion and runoff); turbidity is expected to exceed guidelines during excavated rock placement, however, turbidity and total suspended solids (TSS) are expected to return to baseline 8 months following placement

  •  the decline in reservoir water quality due to sediment disruption and mixing of water between reservoirs during operation

  • infrastructure will have to be built on flood-prone land

Mitigation

Mitigation of the project includes the following:

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  • pre-grouting and segmental lining to reduce tunnel groundwater inflow (fractures and fissures will be dealt with as needed), which may be able to reduce inflow to less than 20% of predicted unmitigated inflow

  • treatment of process water and wastewater prior to discharge to reservoirs

  • discharge of process water and wastewater into reservoirs rather than natural watercourses

  • project elements will be built 40 m away from waterways

  • building away from flood-prone areas when possible, mitigating flood risks and building flood-resistant infrastructure otherwise

  • armoured or encapsulated reservoir rock emplacements with silt curtains to reduce the loss of fine material

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All other mitigation entails monitoring or water management plans and programs that have yet to be developed. No residual or cumulative effects were noted by the report, other than the model-predicted impacts noted above.

Critique

The water quality monitoring as described in the EIS seems robust, however, there are some issues with how the report has assessed impacts. 

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Firstly, as all water-related impacts for this project were determined via simulation and models, it is difficult to go into detail on these impacts and thus certain aspects of the water-related impact assessment appear rather vague. However, as this project is considered to be the first of its kind, the assessment can’t necessarily refer to other projects for data. As the true permeability, fractures, fissures and water inflow that the tunnel will encounter are completely unknown, and this project is unique, the developed mitigation and management may be under or overestimating the impacts to Valued Ecosystem Components significantly.

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For example, the assessment does not go into detail on the downstream effects of process water discharge, wastewater discharge, and excavated rock emplacement into the reservoirs, or how this discharge, while preferable to discharge into natural waterways, will impact the endangered species living within the reservoirs. It also does not mention any cumulative effects. 

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If in accordance with model guidelines, it would have been preferable to see how predicted impacts are reduced when mitigation and management methods were applied to simulation models. Along this same vein, the assessment was confusing as it described the simulation models as both ‘conservative estimation’ and ‘overestimation’ of the project impacts, which are not synonymous terms.

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As for the flooding risk assessment, the report unfortunately focuses on how flooding may impact the project (having to build project infrastructure in flood-prone areas) rather than how the project may affect flooding (how flooding regimes may change during construction and operation and how this may impact water quality or other aspects of the waterways surrounding the project), which is a major flaw. 

Images pertaining to project water impacts.

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